1. Field of the Invention
The present invention relates to an ignition timing control device which electronically controls the ignition timing of an engine.
2. Discussion of Background
FIGS. 9 to 12 show a prior art electronic ignition timing control device disclosed, for example, in Japanese Examined Patent Publication No. 37457/1986. In these drawings, numeral 1 denotes a crankshaft of a four-stroke-cycle four-cylinder engine, and numeral 2 is a circular disc fixedly mounted on the crankshaft and rotating with the rotation of the crankshaft. On the circumference of this circular disc are fixed magnetic bodies 3A and 3B disposed 180 degrees apart from each other. Numeral 4 represents an electromagnetic pickup which is disposed in the vicinity of the outer periphery of the circular disc 2 and produces a reference position pulse P (P.sub.1, P.sub.2, P.sub.3, . . . ) each time it faces the electromagnetic bodies 3A and 3B. This pickup is so constituted that the reference position of the engine crankshaft set at the upper deadpoint of the engine is detected every 180 degrees of rotation of the crankshaft 1.
Furthermore, numeral 5 indicates an oscillator which outputs a clock pulse CP. Numeral 6 is a period measuring means which measures pulse spacing T (T.sub.1, T.sub.2, . . . ) Of the reference position pulse P in time sequence on the basis of the clock pulse CP produced by the oscillator 5. Numeral 7 is a period memory means which, when the reference position pulse P is inputted, stores in a memory a period measured by the period measuring means 6 when a preceding reference position is inputted. Numeral 8 represents an acceleration-deceleration corresponding time output means for determining the value .DELTA.T corresponding to the accelerated speed of engine by deducting a period stored in the period memory means 7, from the period measured by the period measuring means 6. Numeral 9 is an ignition timing computing means which calculates out a spark advance angle .theta. on the basis of the reference crank position to be detected by the electromagnetic pickup 4 in accordance with information S such as engine speed and manifold pressure. And numeral 10 is an ignition time computing means which inputs the measured period T, the acceleration-deceleration corresponding time .DELTA.T, and the value of spark advance .theta., and predictively calculates and outputs a time interval TS from the output of the reference position pulse P until the output of an ignition command signal PS, simultaneously with the reference position pulse P.sub.3, by a method described later. A first ignition command output means 11 to which the time interval TS outputted from this ignition time computing means 10, the clock pulse CP, and the reference position pulse P are inputted, is designed to give out the ignition command signal PS at the time interval TS after the reference position pulse P is outputted. Receiving this ignition command signal PS, an ignition device 12 operates
Now, let the reference position pulse P produced from the electromagnetic pickup 4 be P.sub.1, P.sub.2, P.sub.3, and P.sub.4 in order of generation FIG. 10a, and the period measuring means 6, based on the clock pulse CP of the reference oscillator 5, measures, in sequence of time, the pules period T.sub.1 of the reference position pulse P.sub.1 and P.sub.2 at the time of input of the reference position pulse P.sub.2, and the pulse period T.sub.2 of P.sub.2 and P.sub.3 at the time of input of the reference position pulse P.sub.3. The period memory means 7 functions to store, at the time of input of the reference position pulse P.sub.3, the period T.sub.1 which was measured by the period measuring means 6, for example, when the reference position pulse P.sub.2 is inputted.
Next, let P.sub.4 be the reference position pulse P given out after the reference position pulse P.sub.3, and the period Tf from the reference position pulse P.sub.3 to P.sub.4 is predictively computed by the ignition time computing means 10 immediately after the generation of the reference position pulse P.sub.3.
The predictive computation of the period Tf is performed as described below.
When the engine is running at a fixed speed, the measured period T.sub.2 =the stored period T.sub.1, and accordingly the subsequent period Tf may be predicted to be fixed as EQU Tf=T.sub.2. . . . . . (1)
When the engine speed is not fixed as during acceleration or during deceleration, the acceleration-deceleration corresponding time .DELTA.T produced by variation in the engine speed is predicted as EQU Tf=T.sub.2 -.DELTA.T. . . . . . (2)
For the acceleration-deceleration corresponding time .DELTA.T, the time given by the following formula (3) from, for example, the memory period T1 and the measurement period T.sub.2 is used. EQU .DELTA.T=T.sub.1 -T.sub.1. . . . . . (3)
From the predictive period Tf and the value of spark advance angle .theta., the time interval TS till the following firing time can be given by the following formula. (FIG. 10a) ##EQU1##
As previously stated, engine ignition takes place after the lapse of the time TS given by the formula (4) on the basis of the generation of the reference position pulse. (PS.sub.3 in FIG. 10b).
The ignition timing control device shown in FIG. 1 accurately predicts the period Tf by the use of the formula (1) or (2) when the engine is operating at a constant speed or during continuous acceleration or during continuous deceleration, and, consequently, also accurately controls the ignition timing.
In actual vehicle operation, however, abnormal variation in engine speed sometimes occurs not only during the above described constant-speed engine operation, continuous acceleration or continuous deceleration but also in case of clutch misoperation or abrupt starting of vehicle by a beginning driver.
This abnormal variation in the engine speed is caused rather by mechanical motion of the vehicle body, suspension, engine mounting, and so forth than by the burning of the mixture in cylinders. There exists no correlation between the engine speed and the variation in the engine speed. And the direction of the variation is not fixed and moreover the amount of the variation is very large. It is, therefore, difficult to predict the period Tf from the rotational period of the engine.
FIG. 11 shows an example of such an abnormal variation in the engine speed experimentally created in an actual engine.
This experiment was conducted using a car mounted with a four-stroke-cycle, four-cylinder 2,000 cc engine.
The engine speed indicated when the clutch was suddenly connected, with the engine started from a standstill and raced up to 2,000 rpm and with the transmission shifted into the second-speed gear, was recorded. After the connection of the clutch, the engine speed indicated a variation within the range of from about 200 rpm to about 800 rpm.
A part of the state of generation of the reference position pulse P during this speed variation is shown in FIG. 12a. From a system shown in FIG. 12a, the period Tf can be predicted on the basis of the formula (2): Tf=37 (ms) from T.sub.1 =69 (ms) and T.sub.2 =53 (ms). The actual time interval T.sub.3, however, is 65 (ms), which largely differs from a predicted value.
Therefore, if ignition control is made at this time interval TS computed from this predicted period Tf, the time interval TS when the spark advance angle .theta. is 0.degree., will be TS=37 (ms) from the formula (4), and accordingly, ignition takes place 37 (ms) after the input of the reference position pulse P.sub.3.
That is, the ignition occurs at a position advanced about (65-37)/65.times.180=78.degree. as compared with a target ignition timing. (PS.sub.3 in FIG. 12b)
If the length of the period T.sub.1 and T.sub.2 is opposite to that in FIG. 12a, there will occur an abnormal retardation of spark angle which is not illustrated here.
Particularly, in the event of abnormally advanced ignition timing, excessive knocking will take place or the engine will stop because of the occurrence of a counter torque. If this abnormal speed variation continues, the internal pressure of cylinders of the engine will excessively rise, resulting, in the worst case, in engine trouble.